They're so quiet they're dangerous. That's one of the problems the automotive industry will have to overcome for its all-electric vehicles and hybrids that often run on electric power only.

The National Highway Traffic Safety Administration (NHTSA) said Monday that it is proposing minimum sound standards for hybrid and electric vehicles as a way to make pedestrians more aware of them as they approach.

Because those vehicles don't rely on traditional gas or diesel-powered engines at low speeds, they tend to be much quieter, making them hard to hear given the ambient street noise, says the Detroit Free Press. That's why The proposed standard -- mandated in the Pedestrian Safety Enhancement Act -- would require that the vehicles are detectable in a wide range of street noises whenever traveling under 18 miles per hour.

At 18 miles per hour and faster, the vehicles apparently make enough noise that pedestrians and bicyclists can hear them without added sound.

NHTSA said every automaker will have "a significant range of choices" about what sounds it picks for its vehicles, as long as it meets certain minimum requirements. The agency is sending its proposal to be listed in the Federal Register and the public will have 60 days to comment.

They even put out a series of sample sounds, which seem to sound a lot like other vehicles.

You can hear them at NHTSA-approved Sample Sounds.

The regulation requires that whatever sound is used, it increase in volume and pitch depending on the speed. And hybrid and electric vehicles will need some kind of sound when they are idling and in reverse as well as moving forward, NHTSA said.

If the proposed rule is finalized in a timely fashion, a 3-year phase-in could begin in September 2015.

"Safety is our highest priority, and this proposal will help keep everyone using our nation’s streets and roadways safe," said U.S. Transportation Secretary Ray LaHood.

Always be aware of your surroundings is what I say. Look at trains and how much noise they make, then some idiot walks down the tracks with a headset on and gets hit. Now what kind of thinking is this?

I will not buy an battery hybrid or electric car. Although electric hybrids are well proven, electric hybrid technology is limited in several ways:

1. Cost. The cost of second generation batteries that can charge faster, go farther, and last longer are currently prohibitively expensive. Chevrolet has rumored that the Volt battery pack currently costs more than $16,000. Tesla has indicated its battery pack is over $40,000. Both battery systems are rated at 500 cycles—or about 18 months of daily charging before replacement is necessary. Equivalent hydraulic pump/motor and tanks cost $2,500. 2. Charge time. Charge time not only impacts overall efficiency and utilization, but also how efficient the brake regeneration system is. Toyota currently estimates their brake regeneration system at 30 percent efficiency, while hydraulic brake energy regeneration efficiency is over 80 percent, since hydraulics are not limited by battery charge limits during the regeneration cycle. 3. Battery Weight. Weight has a dramatic impact on overall transportation efficiency, and even lithium ion battery technologies add significant weight. Toyota’s current battery pack adds 150 pounds to the Prius, the Chevy Volt battery pack is expected to weigh 350 pounds, and Tesla’s battery pack exceeds 1000 pounds. Equivalent storage in carbon fiber tanks weigh 75 pounds. 4. Maintenance talent availability. Electric hybrid vehicles are highly complex and the DC power can be dangerous if not handled by specifically trained mechanics. There are not a large number of specifically trained mechanics in the US today, and the training requirements are significant. Most fleet mechanics already have training and a strong comfort level with hydraulic technology.

With the current electric hybrid limitations in mind, and recognizing that there are currently no competitors serving the light- and medium-duty hydraulic hybrid space in the automotive or retrofit markets, HHS has designed a Hydraulic Hybrid System for the automotive and fleet vehicle retrofit markets.